This invention relates to a differential pressure transducer for the measurement of pressure difference between two isolated or separately controlled spaces, rooms, or chambers where air (or any other gas) is nearly at the same pressure in the two spaces but small differences are important to measure.
To measure the small pressure difference, a small tube or passageway is connected between the two spaces so that the gas flows in the tube from the higher pressure to the lower pressure chamber. The flow rate will be proportional to the pressure difference in accordance with principles expounded by Bernoulli. Two thermal sensors similar to hot wire or hot film anemometer sensors are mounted inside the tube or passageway at a location where such tube is horizontal and in such a way that one sensor is always directly upstream from the other.
Sensing instruments known as hot wire or hot film anemometers have been developed for measuring fluid flow parameters such as velocity and mass rate of flow. They are particularly applicable for this device because their sensitivity increases at lower flow rates making ideal sensors for measuring small differential pressures. Example of this type of anemometer and circuits therefor are shown in U.S. Pat. Nos. 3,333,070, 3,352,154, 3,592,055, 3,604,261, 3,677,085, 3,900,819, 3,995,481, 4,024,761, 4,206,638, 4,279,147.
When a single thermal sensor is mounted in a tube, it can sense the velocity of flow but typically cannot sense the direction of flow. If the higher pressure chamber is always the same so that the direction of flow is known, then the problem of determining variations in flow direction does not exist. However, when the higher pressure changes from side to side so the direction of flow in the flow measuring tube changes, it is necessary to have an indication of flow direction before the real pressure in either of the chambers is known relative to the other. This invention relates to a method for measuring the pressure difference with the direction or sign (plus or minus) known at all times even when the pressure difference is very small.
A thermal sensor is heated electrically and controlled by the hot wire anemometer circuit to be self heated at an elevated temperatue relative to the flow stream. Hence, when flow passes over the heated sensor, it heats the air (or gas) as it goes by, creating a thermal wake or heated portion of air. If another similar sensor is located such that it is in the thermal wake of the first sensor, the temperature difference that this second sensor encounters is altered because the air passing over it is a higher temperature. It is important that the flow tube be horizontal to avoid discrepancies due to free convention flow generated by thermal sensors at very low flow rates. When a means is determined to indicate which sensor is experiencing the wake of the other sensor, then the direction of flow is known. Then also, the upstream unaffected sensor sensor can be selected for the flow measurement to sense the pressure difference.
It is the object of the present invention to provide a novel and improved pressure transducer that overcomes the prior problems of sensing direction in a pressure transducer that utilizes heated sensors in a flow between two spaces.
It is another object of this invention to provide a directional pressure transducer utilizing thermal sensing in flow tube with the thermal wake of one heated sensor affecting a second heated sensor to provide a directional sensor particularly at low velocities. This can be done if there are no obstructions or connecting bodies between the two sensors that disturb the thermal wake as in other previous directional thermal sensors.
It is another object of this invention to provide a novel and improved pressure transducer that employs a flow tube with two similar thermal sensors mounted within where such sensors are heated metal, core wound wire, surface film, or other electrically-heated resistance elements of various types of shapes that result in the first sensor generating a significant thermal wake to be sensed by the second similar sensor when properly aligned in a downstream direction. It is important that the thermal sensors are mounted so they are suspended in the flow stream so that the fluid flows completely around or almost completely around the thermal sensors to optimize the generation of the thermal wake to be sensed by the other similar thermal sensor. This pair of thermal sensors are connected in series and are electrically heated and controlled so that their total series resistance is maintained constant by feedback operation. Then the voltage at the junction of the two sensors can be monitored to indicate a change in value which in turn indicates direction while the total series output of the two thermal sensors is electrically monitored to indicate flow. Alternately, this pair of thermal sensors are controlled individually by two separate hot wire anemometer type circuits where the electrical outputs are compared to sense which sensor has sensed the thermal wake and has experienced a shift in calibration, and thereby, indicating direction.